Not sure what's going on; I tried four different browsers across two separate computers, and it was basically impossible for me to log on and try to find award travel.
Maybe this has to do with some DNS settings or some router extension, but the network is pretty standard, and at least one of those browsers was entirely "virgin" (no extensions/addons).

I had the same problem with Lufthansa - the website was basically unusable to log in and find flights. Delta's website showed the connections fine.
I tried finding a way to contact Flying Blue/Air France/KLM on their website, but, guess what, that also didn't work lol

They really need to fix their overcomplicated tracking-laced cross-site-script dependent websites, because it's interfering with basic necessary functionality for real tasks.

I also want to preface any replies "I don't know it works for me" - that's not a helpful or useful thing to post.

Have been looking for award flights this evening and had no issues. I'd say this it is rather important to know that the website works, as it points to a problem on your side.
What exactly is not working? What does it mean when you say "impossible to log on"? An error? Slow? Something else?

I just tried to log in via airfrance.us. This took me to https://identity.airfranceklm.com/login/otp even though I was already logged in at flyingblue.com. I enter my e-mail, push "continue" and nothing happens. I get an error " Sorry, an unexpected technical error occurred. Please try again or contact the Air France customer service team.".

On a different browser: https://wwws.airfrance.us logs me in, then I go to "book with miles". I select a flight, and I get taken back to the booking screen. When I push back on the browser, I get: "Sorry, something went wrong... Please try again." When I do try again, it seems to work, at least this time when I tried. This was the first time I was able to get through to the end of the booking. Other times it did not. I wish I could give more details.

When I had this it was a me problem and I had to clear all cookies and cache etc

website and app have been working well all day for me

Good call. I think I tried a private browsing window on some browsers.... still didn't work

Yeah I had the same, really got to delete all.
For me the problem pops up the most with the FlyingBlue website

This is something that really needs sorting out, and not in an entitled sense. Booking flights is already stressful enough.

Deleted - now realise the OP is seeking award tickets.

Have you tried using the AF or KL app?

I've had errors like this multiple times when trying to log in, in incognito/private mode. Another "error" when private mode was toggled was that calendars with fares for different days/destinations has not been showing at all (even if you'd pick specific dates). The pages just keeps loading forever and never shows you any prices...

Seems that KL/AF/Flying Blue really wants to track and save the history of our searches, so that they can jack the prices up even more....